Water-based ink for ink jet recording is used generally in the form of a water-soluble dye solution or a liquid obtained by dispersing a water-insoluble color material into water or an aqueous solution including a water-soluble organic solvent. On the other hand, ink for ink jet recording that includes a sublimation dye has been used for sublimation transfer dyeing. For example, there is a method in which the ink including a sublimation dye is printed by ink jet printing on a sheet medium such as paper, and this medium is then put on a hydrophobic fiber product or resin film product for sublimation transfer. There is also another method, as disclosed, e.g., by JP 2002-79751 A, in which the ink including a sublimation dye is printed by ink jet printing on a film product that has a releasable ink receptor layer and can be used for sublimation and diffusion dyeing, the film product is then heated so that the lower layer is dyed by sublimation and diffusion, and the ink receptor layer is removed.
The sublimation dye ink is used generally in the form of a liquid obtained by dispersing the sublimation dye into an aqueous solution including water, a water-soluble organic solvent, and a dispersant. However, the water-soluble organic solvent is evaporated with water by heat treatment, e.g., when the ink is printed on the paper medium by ink jet printing, the medium is then put on a transfer object, and the object is dyed by sublimation transfer while heating, or when the ink is printed on the releasable ink receptor layer of the film product that can be used for sublimation and diffusion dyeing, the film product is then heated to sublimate and diffuse the sublimation dye so that the lower layer of the film product is dyed. This is the point different from ink jet printing that is used only for general printing purposes. The sublimation transfer dyeing method requires heating for a predetermined time at temperatures higher than the sublimation point of the sublimation dye. Therefore, even a high-boiling organic solvent may be evaporated. Particularly for a high-boiling organic solvent such as glycerin, the temperature is reduced to a condensation point shortly after the evaporated solvent is released into the atmosphere, thus causing smoky steam. This is considered to be undesirable because the appearance becomes extremely worse. One possibility for avoiding such a problem is to use a low-boiling solvent that does not cause any smoky steam. However, the low-boiling solvent is the same as the high-boiling organic solvent in release of the evaporated solvent into the atmosphere. In either case, therefore, environmental pollution (e.g., working environment) is a problem.
Characteristics required for ink for ink jet recording are as follows: (1) ejection stability while preventing nozzle clogging; (2) storage stability; (3) high-quality recording images resulting from uniformity in the direction of ink ejected, the ejection amount, and the dot shape; (4) quick drying and fixation; and (5) high print density.
In an ink jet recording system, ink should be ejected from thin nozzles as ink droplets. Therefore, (1) is particularly important. When the ink jet recording system is of the drop-on-demand type, the ejection of ink from the nozzles is stopped temporarily even during continuous operation. This may lead to an ejection failure in long continuous operation.
Unlike a water-soluble dye, the ink for sublimation transfer ink jet recording that includes a sublimation dye tends to cause clogging, e.g., because precipitates (aggregates) are generated by the degradation of dispersion stability and moisture is evaporated from the nozzles. In particular, when the apparatus is not operated for a long time, clogging is likely to occur due to evaporation of moisture.
Therefore, a humectant is added to suppress the evaporation of moisture. Examples of the humectant include a high-boiling organic solvent of glycols such as glycerin or polyethylene glycol, and saccharides.
As described above, however, the sublimation transfer method requires heating for a predetermined time at temperatures higher than the sublimation point of the sublimation dye. Therefore, even if a high-boiling organic solvent is used as a humectant, the solvent is evaporated at a heating temperature during sublimation transfer, resulting in environmental pollution. Thus, it is necessary to use a humectant that is not an organic solvent, is not evaporated by heating in sublimation transfer, and has no adverse effect on the characteristics (1) to (5) required for the ink.
As an example of a humectant other than a water-soluble organic solvent, JP 57(1982)-57762 A discloses a water-based inkjet ink including a water-soluble dye and sorbitol. JP 60(1985)-72968 A and JP 62(1987)-101672 A disclose a water-based pigment ink including pigment and sorbitol. JP 2(1990)-214785 A discloses a water-based ink for ball point pen that includes maltitol and uses a water-soluble dye or pigment as a colorant. JP 2001-115070 A discloses that xylitol and a water-soluble organic solvent are used for carbon black dispersions.
In the above conventional techniques, sugar alcohol has been used as a humectant for a pigment ink or water-soluble dye ink. However, none of the conventional techniques disclose whether sugar alcohol can be used as a humectant for an ink in which a sublimation dye is dispersed with a dispersant. Moreover, no study has been conducted on a sublimation dye ink that includes the smallest possible amount of water-soluble organic solvent in consideration of environment, and no technique has sought to find a solution to a problem that may arise with that case. Further, sugar alcohol should be used as a humectant while taking into account the fact that it has no adverse effect (e.g., oxidation-reduction) on the sublimation dye.
JP 2000-265098 A discloses an inkjet ink that includes water, a colorant, and a compound expressed by the following chemical formula (II):R—A—(CH2CH2O)x—H  (II)where R is linear saturated hydrocarbon having a carbon number of 14 to 23, A is O or OCO, and x is an integer of 1 to 3. This ink may include a humectant as needed.
However, JP 2000-265098 A discloses “various dyes, pigment, or colored polymer/wax can be used as a colorant, and a water-soluble dye is particularly preferred. Any type of water-soluble dye, e.g., . . . can be used, and an acid dye and direct dye are particularly preferred.” In this document, only acid dye is used actually in the example. Therefore, as is evident from the description, JP 2000-265098 A does not disclose the use of the compound expressed by the chemical formula (II) in a sublimation dye ink. Even if R is polyoxyethylene higher alkyl ethers having a carbon number of not more than 23, it is not possible to provide an ink composition with a high redispersion property when a sublimation dye is used as a colorant.
To solve the above problems of a conventional ink for ink jet recording that includes a sublimation dye, an object of the present invention is to provide an ink for ink jet recording that not only can reduce environmental pollution and maintain required performances such as storage stability and high-quality recording images, but also can effectively prevent nozzle clogging, exhibit good dispersion stability, and achieve a high redispersion property that ensures stable ejection both during long continuous operation and after stopping the operation of an apparatus for a long time. Another object of the present invention is to provide a sublimation transfer dyeing method using this ink.